Coated products have found uses in a wide variety of fields and have been produced by numerous processes. As used herein, a coated product is one in which a substrate is covered predominantly on a surface with one or more materials to give the substrate a property it does not possess by itself. These properties may include, but need not be limited to, chemical, physical, electrical, optical, and aesthetic. A coating is considered continuous when it imparts the prerequisite desired property to the coated product. For the purpose of this patent, continuous is thus defined in a functional sense. For example, if the coating is to provide the property of being waterproof, then it will be considered continuous if, under defined test conditions, it does not allow liquid water penetration. Similarly, depending on any other desired function, a coating will be considered continuous if it provides the desired properties, e.g. electrical conductivity, abrasion resistance, opacity, associated aesthetics or such other properties as defined by either an appropriate test method or end-use characterization. Certain applications require that the coating be free of pin-holes. One method of determining this is to subject the coating in question to a low pressure hydrostatic water challenge as described herein. If the coating passes this challenge it is considered to be continuous.
A useful summary of coating processes is found in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 6, pages 387-426 (1979 Wiley). Additional useful information may be found also in volume 10, pages 216-246.
Continuous coatings have been produced by two types of film forming processes:
(1) The formation of the film and attachment of the film to the substrate occur simultaneously; and
(2) The film is formed independently, followed by attachment to the substrate, in separate distinct stages.
Coating techniques wherein film forming and combination with the substrate occur simultaneously are universally characterized by the exertion of hydraulic pressure by the liquid coating. This pressure which causes the film formation also causes penetration of the coating into the substrate which can lead to a non-supple, poor drape product. Where this is not desirable, a delicate balance must be maintained through control of the variables, including the incoming substrate, the coating rheology and surface tension, and the coating station design and operation. Such methods are undesirable as they require extensive operator control, are difficult to run, and thus may be operationally expensive.
Controlling the penetration of the substrate by the liquid coating while causing quality film formation can be achieved by optimizing all variables, which is at best a compromise of these variables. For instance, the substrate may be modified to cause more resistance to coating penetration. However, the modifications, known in the art, compromise properties such as adhesion of the coating, decreased drape and suppleness of the substrate, lost permeability through the substrate, and the added cost of additional processing steps prior to coating. These modifications in substrate are generally made and further compromises a range of substrates already limited in selection by those having appropriate characteristics facilitating the above coating methods.
The art teaches modification in the coating via controlling the rheology and surface tension to prevent penetration of the substrate. The range of acceptable coatings is thus further narrowed as these compromises may have deleterious results such as decreased adhesion.
Fortunately, the art has been developed to allow for a variety of techniques which have opened the window for a broader range of substrates and coatings of various chemistries that can be used for coated products by this family of coating techniques. In general, these coating processes have been designed to control and minimize the hydraulic forces causing the film penetration into the substrate. However, a balancing of interactions is still required to various degrees and, as stated, frequently results in compromises being made. Thus, coated products made by the techniques of film forming and simultaneous combination with the substrate are limited in scope to the tighter, smoother substrates and to those coating chemistries that allow for a medium range of viscosities.
These coating techniques further limit the possible substrates when a thin, continuous coating is desired. If the substrate has any substantial texture to it, then in order to insure continuity, thicker coatings must be applied or vice versa. If a thin coating is desired for functional reasons, a smooth substrate must be selected. Furthermore, an open substrate proves very difficult, if not impossible to coat.
More versatility in selection of the substrate is allowed when film formation is done prior to combination with the substrate. These processes, however, require specific rheological characteristics of the chemistry employed, which limit the candidate coating chemistries utilized. Furthermore, these processes require expensive equipment and are more frequently seen in volume-driven businesses and products. The performance requirements of the equipment and the chemistry become increasingly more exacting as the thickness of the coating is decreased. In addition, these processes still exhibit a tradeoff between adhesion of the coating to the substrate and penetration of the coating into the substrate.
Films formed by these techniques are frequently laminated to the substrate, rather than combined directly after film forming. This is particularly true where the films are desired to be thin and continuous so as to avoid any disruption of the integrity of the film. Lamination, as the term is used in this patent, requires having films formed in isolation and subsequently affixing them to a substrate. Particularly in thin, continuous film requirements, films are frequently affixed to the substrate by a separate adhesive layer. In some cases this affixing step may be achieved by partially remelting, or otherwise reflowing, the surface layer of the film. This approach suffers from the same problems of adhesion, penetration, continuity, and control referred to earlier, particularly with thin films. Lamination furthermore has the deleterious requirement of further processing steps while still necessitating control of penetration of the adhesive layer into the substrate. Attempts, although none totally satisfactory, to open this window of trade offs are well known in the art and effort continues as such.